Transcript Delta modulation DM

Delta modulation DM
DM is a simple modulation scheme used
to transmit one bit per sampling frequency
In DM the analog information signal is
oversampled purposely to increase the
correlation between adjacent samples
This is done to permit the use of a simple
quantization strategy for the construction
of the encoded signal
1
DM principle of operation
In its basic form, DM provides a stair case
approximation to the oversampled version
of the message signal as shown
2
DM principle of operation
The difference between the input signal
sample and the stair case approximation is
quantized into only to two levels  
If the current sample is greater than the
previous sample then the DM modulator
generates  
If the current sample falls below the
previous sample then the modulator
generates  
3
DM discrete-time equations
The discrete time equations which
describes delta modulation are
e[n]  m[n]  mq [n  1]
eq   sgn e[n]
mq [n]  mq [n  1]  eq [n]
4
DM discrete-time equations
Where e[n] is the error signal representing
the difference between the present sample
m[n] and the latest approximation mq[n-1]
to it
eq[n] is the quantized version of the error
5
Main advantage of DM
The main advantage of DM is its simplicity
6
DM modulator
The delta modulator can be constructed
from the discrete time equations according
to the block diagram shown below
7
DM demodulator
The DM demodulator do the reverse
operation of the modulator to reconstruct
the original signal
This can be achieved by summing the
received bit stream as described by the
n
following equation mq [n]   sgn e[i]
i 1
n
  eq [i ]
i 1
8
DM demodulator
The waveform results at the receiver
contains a stair-case approximation
The stair approximation corresponds to
the presence of high frequency distortion
presented in the message signal
The high frequency distortion can be
removed by using a LPF
9
DM demodulator
The DM demodulator can be constructed
from the discrete time equations
describing the demodulator according
block diagram shown below
10
Quantization errors in delta
modulation
Delta modulation is subjected into two
quantization error distortions
1. Slope overload distortion
2. Granular noise
11
Slope overload distortion
Slope overload distortion occurs when the
step size is too small for the stair case
approximation to follow a steep segment
of the message signal m(t)
12
Slope overload distortion
Slope over load distortion can be avoided
if the step size ∆ is selected according to
the following equation
 dm(t )

Ts
dt
13
Granular noise distortion
Granular noise occurs when the step size
∆ is too large relative to the local slope
characteristics of the input signal
This would cause the stair case
approximation mq(t) to hunt around a
relatively flat segment of the input
waveform
14
Disadvantage of DM
The main disadvantage of the DM scheme
is the presence of an accumulative error in
the demodulated signal
This problem can be solved by using a
delta-sigma modulator
15
Delta-sigma modulation
In the delta sigma modulator the input
signal, m(t), will be integrated prior to delta
modulation
The use of integration in the manner
described here has the following beneficial
effects
1. The low frequency content of the input
signal is pre-emphasized
16
Delta sigma modulation
2. Correlation between adjacent samples of
the delta modulator input is increased,
which tends to improve the overall
system performance by reducing the
variance of the error signal at the
quantizer input
3. Design of the receiver is simplified
17
Delta sigma modulator block
diagram
The block diagram of the delta sigma
modulator is shown below
18
Delta sigma modulator block
diagram
The previous block diagram can be greatly
simplified if the signal is first differentiated
then applied to the input of the integrator
19
DM examples
20
DM examples
21
DM examples
22
DM examples
23